Anacom 1-1 1-2
December 24, 2022 | Author: Anonymous | Category: N/A
Short Description
Download Anacom 1-1 1-2...
Description
AT02 Student Workbook
The ANACOM 1/1 and ANACOM 1/2 Boards Chapter 1
Chapter 1 The ANACOM 1/1 and ANACOM 1/2 Boards B oards
1.1 1.1
La Layo you ut D Dia iagr gram am of th thee A ANA NACO COM M 1/ 1/1 1 Bo Boar ard d
Figure 1
LJ Technical Systems
13
The ANACOM 1/1 and ANACOM 1/2 Boards Chapter 1
1. 1.2 2
AT02 Student Workbook
Th Thee AN ANACO COM M1 1//1 B Bo oar ard dB Bllock ckss The transmitter board can be considered as five five separate blocks:
Power Pow er inp input ut
ANACOM 1/1 DSB/SSB AM TRANSMI TRANSMITTER TTER
Antenna
Transmitter output
Audio input
Modulator 15
AUDIO AMPLIFIER
Switched faults
L J
VOLUME
Loudspeaker
HEADPHONE S
Figure 2
1.3
Power Input These are the electrical input connections necessary to power the module. The LJ Technical Systems "IC Power 60" or "System Power 90" are the recommended power supplies. supplies.
+1 2 V
0V
- 12V
Figure 3
14
LJ Technical Systems
AT02 Student Workbook
1.4 1.4
The ANACOM 1/1 and ANACOM 1/2 Boards Chapter 1
Th Thee A Au udi dio o In Inpu putt a an nd A Amp mpli lifi fier er This circuit provides an internally generated signal that is going to be used as 'information' to Input demonstrate operation the transmitter. Thereinformation is also an External Audio facility the to enable us toofsupply our own audio signals. The information signal can be monitored, if required, by switching on the loudspeaker. An amplifier is included to boost the signal power to the loudspeaker.
AUDIO OSCILLATOR OSCILLATOR
AMPLITUDE FREQUENCY
MIN
MAX
MIN
MAX
14
AUDIO INPUT SELECT
EXTERNAL AUDIO INPUT
INT
EXT
16
0V
Figure 4
LJ Technical Systems
15
The ANACOM 1/1 and ANACOM 1/2 Boards Chapter 1
1.5
AT02 Student Workbook
The Modulator This section of the board accepts the information signal and generates the final signal sign al to be transmitted. BALANCED MODULATOR & BANDPASS FILTER CIRCUIT 1
T1 BALANCE
455kHz OSCILLATOR
2
1MHz CRYSTAL OSCILLATOR DSB MODE SSB
7
T2 4
T3
5 8
BALANCED MODULATOR
CERAMIC BANDPASS FILTER
BALANCED MODULATOR & BANDPASS FILTER CIRCUIT 2 BANDPASS
19
21
18
BALANCE
T4
BALANCE
Figure 5
16
LJ Technical Systems
AT02 Student Workbook
1.6
The ANACOM 1/1 and ANACOM 1/2 Boards Chapter 1
The T Tra ran nsm smiitter O Ou utput The purpose of this section is to amplify the modulated signal ready for transmission. transmi Thethe transmitter transm itter output o utput can be connected to the receiver by a screened cable orssion. by using antenna provided. The on-board telescopic antenna should be fully extended to achieve the maximum range of about 4 feet (1.3m). After use, to prevent damage, the antenna should be folded down into the transit clip mounted on the ANACOM board. Antenna
OUTPUT AMPLIFIER
13
ANT.. ANT
12 SKT.
TX OUTPUT SELECT
TX. OUTPUT OUTPUT GAIN
0V
Figure 6
LJ Technical Systems
17
The ANACOM 1/1 and ANACOM 1/2 Boards Chapter 1
1.7
AT02 Student Workbook
The Switched Faults Under the black cover, there are eight switches. These switches can be used to simulate in various partsany of the circuit. The faultsTo areensure normally one at a fault time,conditions but remain safe under conditions of use. thatused the ANACOM 1 boards are fully operational, all switches should be set to OFF. Access to the switches is by use of the key provided. Insert the key and turn counter-clockwise. To replace the cover, turn the key fully clockwise and then slightly counter-clockwise to release the key.
SWITCHED FAULTS
Figure 7
Notes: ..................................................................................................................................... ..................................................................................................................................... ..................................................................................................................................... ..................................................................................................................................... ..................................................................................................................................... ..................................................................................................................................... ..................................................................................................................................... ..................................................................................................................................... ..................................................................................................................................... .....................................................................................................................................
18
LJ Technical Systems
AT02 Student Workbook
1.8 1.8
The ANACOM 1/1 and ANACOM 1/2 Boards Chapter 1
La Layo you ut D Dia iagr gram am of th thee A ANA NACO COM M 1/ 1/2 2 Bo Boar ard d
Figure 8
LJ Technical Systems
19
The ANACOM 1/1 and ANACOM 1/2 Boards Chapter 1
1. 1.9 9
AT02 Student Workbook
Th Thee AN ANACO COM M1 1//2 B Bo oar ard dB Bllock ckss The receiver board can be considered as five separate blocks:
Power Po wer in inpu putt
ANACOM 1/2 DSB/SSB AM RECEIVER
Receiver input
Receiver
Audio output
Switched faults
Figure 9
1.10 Po Pow wer IIn nput These are the electrical input connections necessary to power the module. The LJ Technical Systems "IC Power 60" or "System Power 90" are the recommended power supplies. supplies. If both ANACOM ANACOM 1/1 and ANACOM ANACOM 1/2 boards are to be used, they can be powered by the same power supply unit.
+12V
0V
Figure 10
20
LJ Technical Systems
AT02 Student Workbook
The ANACOM 1/1 and ANACOM 1/2 Boards Chapter 1
1.11 1.11 Th Thee R Rec ecei eive verr IIn nput put In this section the input signals can be connected via a screened cable or by using the provided. antenna should be used fully extended and, afterantenna use, folded down The into into the ttelescopic he transit clip.
RX.
ANT.. ANT
INPUT SELECT
SKT.
RX. INPUT
Figure 11
Notes: ...................................................................................................................................... ...................................................................................................................................... ...................................................................................................................................... ...................................................................................................................................... ...................................................................................................................................... ...................................................................................................................................... ...................................................................................................................................... ......................................................................................................................................
LJ Technical Systems
21
The ANACOM 1/1 and ANACOM 1/2 Boards Chapter 1
AT02 Student Workbook
1.12 The R Reeceive verr The receiver amplifies the incoming signal and extracts the original audio information signal. The incoming originating from ANACOM 1/1. signals can be AM broadcast signals or those
OUT
AGC CIRCUIT 3
4
IN
0V
DIODE DETECTOR 2
1
R.F. R.F. AMPLIFIER
MIXER
I.F. AMPLIFIER 2
I.F. AMPLIFIER 1
31
5 13
TC1 INT
29
T1 6
T2
TUNED CIRCUIT SELECT
14
T3
30
T4
15
12
24
20
28
PRODUCT DETECTOR EXT
GAIN
25 16
7
8
21
17 9
26
10
32
TUNED CIRCUIT INPUTS
23
18
34
37
27
22 33
19
11
40
0V
0V 35
LOCAL OSCILLATOR
36
BEAT FREQUENCY OSCILLATOR
42 41 OFF T6
T5 ON 43
TC2
44
45
TUNING
Figure 12
22
LJ Technical Systems
AT02 Student Workbook
The ANACOM 1/1 and ANACOM 1/2 Boards Chapter 1
1.13 1.13 Th Thee Au Audi dio oO Ou utp tpu ut The information signal from the receiver can be amplified and heard by using a set of headphones or, if required, by the loudspeaker provided. AUDIO AMPLIFIER
SPEAKER OFF 38
39
ON
HEAD PHONES VOLUME
0V
Figure 13
LJ Technical Systems
23
The ANACOM 1/1 and ANACOM 1/2 Boards Chapter 1
AT02 Student Workbook
1. 1.14 14 Th Thee Sw Swit itch ched ed F Fau ault ltss Under the cover, there are eight switches. These switches can be used to simulate fault various the circuit. are normally one at a1 time, conditions but remaininsafe underparts any of conditions of The use. faults To ensure that the used ANACOM boards are fully fully operational, all switches should be set to OFF. Access to the switches is by use of the key provided. Insert the key and turn counter-clockwise. To replace the cover, turn the key fully clockwise and then slightly counterclockwise clockwi se to release the key.
SWITCHED FAULTS
Figure 14
Notes: ..................................................................................................................................... ..................................................................................................................................... ..................................................................................................................................... ..................................................................................................................................... ..................................................................................................................................... ..................................................................................................................................... ..................................................................................................................................... ..................................................................................................................................... ..................................................................................................................................... .....................................................................................................................................
24
LJ Technical Systems
AT02 Student Workbook
An Introduction to Amplitude Modulation Chapter 2
Chapter 2 An Introduction to Amplitude Modulation
2.1 2.1
Th Thee Fr Freq equ uen ency cy Com Compo pone nen nts o off th thee Hu Huma man n Vo Voice ice When we speak, we generate a sound that is very complex and changes continuously so at a particular instant in time the waveform may appear as shown in Figure 15 below. However complicated the waveform looks, we can show that it is made of many different sinusoidal signals added together.
Amplitude time
Figure 15
To record this information we have a choice of three methods. The first is to show the original waveform as we did in Figure 15. The second method is to make a list of all the separate sinusoidal waveforms that were contained within the complex waveform (these are called 'components', or 'frequency components'). This can be seen in Figure 16 overleaf.
LJ Technical Systems
25
An Introduction to Amplitude Modulation Chapter 2
AT02 Student Workbook
Only four of the components of the audio signal in Figure 15 are show shown n abov above. e. The The actu actual al numb number er of comp compon onen entts de depe pend ndss on th thee sh shap apee of the signal being consi sid dered and coul ould be a hundr ndred or more if the wavefo wav eform rm was ve very ry co comp mple lex. x. Figure 16
The third way is to display all the information on a diagram. Such a diagram shows the frequency spectrum. It is a graph with amplitude plotted against frequency. Each separate frequency is represented by a single vertical line, the length of which represents the amplitude of the sinewave. Such a diagram is shown in Figure 17 opposite. Note that nearly all speech information is contained within the frequency range of 300Hz to 3.4kHz.
26
LJ Technical Systems
AT02 Student Workbook
An Introduction to Amplitude Modulation Chapter 2
Amplitude
0
300Hz
3.4kHz
Frequency
Figure 17 A Typical Voice-Frequency Spectrum
Although an oscilloscope will only show the original complex waveform, it is important for us to remember that we are really dealing with a group of sinewaves of differing frequencies, amplitudes and phases.
2.2 2.2
AS Sim impl plee C Co ommu mun nicat icatiion Sy Syst stem em Once we are out of shouting range of another person, we must rely on some communication system to enable us to pass information. The only essential parts of any communication system are a transmitter, a communication link and a receiver, and in the case of speech, this can be achieved by a length length of cable with a microphone microphone and an amplif amplifier ier at one end and a loudspeaker and an amplifier at the other.
Amplifier Microphone
Communicat Commu nication ion link (a wi wire re in th this is ex exam ampl plee)
Amplifier Loudspeaker Figure 18 A Simple Communication System
For long distances, or for when it is required to send signals to many destinations at the same time, it is convenient to use a radio communication system. LJ Technical Systems
27
An Introduction to Amplitude Modulation Chapter 2
2.3
AT02 Student Workbook
The F Frrequency P Prroblem To communicate by radio over long distances we have to send a signal between two antennas, one at the t he sending sending or transmi t ransmitting tting end and the other ot her at the receiver.
Antenna
Antenna
Transmitter
Receiver
Figure 19
The frequencies used by radio systems for AM transmissions are between 200kHz and 25MHz. A typical radio frequency of, say, 1MHz is much higher than the frequencies present in the human human voice. We appear to have two incompatible requirements. The radio system uses frequencies like 1MHz to transmit over long distances, but we wish to send voice frequencies of between 300Hz and 3.4kHz that are quite impossible to transmit by radio signals.
28
LJ Technical Systems
AT02 Student Workbook
2.4
An Introduction to Amplitude Modulation Chapter 2
Modulation This problem can be overcome by using a process called 'modulation'. The radio system can easily send high frequency signals between a transmitter and a receiver but this, on its own, conveys no information. Now, if we were to switch it on and off for certain in intervals, tervals, we could use it to send information. For example, we could switch it on briefly at exactly one second intervals and provide a time signal (see Figure 20 below). Messages could be passed by switching switching it on and off in a sequence of long and short bursts and hence send a message by Morse Code. Figure 20 below shows the sequence that would send the distress signal SOS.
Onee secon On second d int interv erval al A ti time me sign signal al
An SOS SOS di dist stre ress ss sign signal al Figure 20
The high frequency signal that has been used to send or 'carry' the information from one place to another is called a 'carrier wave'. The carrier wave must be persuaded in some way to convey the speech to the receiver. The speech signal represents the 'information' that we wish to send and therefore this signal is called the 'information signal'. The method employed is to change some characteristic of the carrier wave in sympathy with the information signal and then, by detecting this change, be able to recover the information signal at the receiver. LJ Technical Systems
29
An Introduction to Amplitude Modulation Chapter 2
2. 2.5 5
AT02 Student Workbook
Ampli plitude Mo Mod dulat atiion ((A AM) The method that we are going to use is called Amplitude Modulation. As the name would suggest, we are going to use the information signal to control the amplitude of the carrier wave. As the information signal increases in amplitude, the carrier wave is also made to increase in amplitude. Likewise, as the information signal decreases, then the carrier amplitude ampl itude decreases. By looking at Figure 21 below, we can see that the modulated carrier wave does appear to ‘contain’ in some way the information as well as the carrier. We will see later how the receiver is able to extract the information from the amplitude modulated modul ated carrier wave.
Informa Information tion sig signal nal
Amplitude Modulator
Modulated car carrier rier wave
Carri Car rier er wa wave ve in input put
Figure 21
2.6
Depth o off M Mo odulation The amount by which the amplitude of the carrier wave increases and decreases depends on the amplitude of the information signal and is called the 'depth of modulation'. The depth of modulation modulation can be quoted as a fraction fract ion or as a percentage. Percentage modulation =
30
Vm ax − V min
V max + V min
× 100% LJ Technical Systems
AT02 Student Workbook
An Introduction to Amplitude Modulation Chapter 2
Here is an example:
0V
6V
10V
Vmin Vmax Figure 22 Depth of Modulation Modulat ion
In Figure 22 we can see that the modulated carrier wave varies from a maximum peak-to-peak value of 10 volts, volts, down to a minim minimum um value value of 6 volts. volts. Inserting these figures in the above formula, we get: Percentage modulation
= 10 − 6 × 100% 10+6 = =
2.7
4
× 100% 16 25% or 0.25
The F Frrequency S Sp pectrum Assume a carrier frequency (f c) of 1MHz and an amplitude of, say, 5 volts peak-to peak. The carrier could be shown as:
5V
Amplitude
0
Carrier
1MHz
Frequency
Figure 23 The Frequency Spectrum Spec trum of a Carrier Wave
LJ Technical Systems
31
An Introduction to Amplitude Modulation Chapter 2
AT02 Student Workbook
If we also have a 1kHz information signal, or modulating frequency (fm), with an amplitude of 2V peak-to-peak it would look like this:
5V Amplitude Carrier
2V Informa Info rmati tion on Si Signal gnal 0 1kHz
Frequency
1MHz
Figure 24 The Frequency Spectrum of a Carrier Wave and an Information Signal
When both signals have passed through the amplitude modulator they are combined to produce an amplitude amplitude modulated modulated wave. The resultant AM signal has a new frequency spectrum as shown in Figure 25 below: bel ow:
Carr er
5V Amplitude Lowe Lo werr Si Side de Fr Freq eque uenc ncy y
2V
0
Uppe Up perr Side Side Fr Frequ equen ency cy
Frequency
Notice that the1kHz signal is no longer present Figure 25 Frequency Spectrum of Resultant Re sultant AM Sign Signal al
32
LJ Technical Systems
AT02 Student Workbook
An Introduction to Amplitude Modulation Chapter 2
Some interesting interesting changes have occurred as a result of the modulation process. (i) The original 1kHz information frequency has disappeared. (ii) The 1MHz carrier is still present and is unaltered. (iii)) There are two (iii t wo new components: Carrier frequency (f c) plus the information frequency, called the upper side frequency (f c + f m) and Carrier frequency (f c) minus the information frequency, called the lower side frequency (f c - f m) The resulting signal in this example has a maximum frequency of 1001kHz and a minimum min imum frequency of 999kHz and so it occupies a range of 2kHz. This is call called ed the bandwidth of the signal. Notice how the bandwidth is twice the highest frequency contained in the information signal.
Notes: ...................................................................................................................................... ...................................................................................................................................... ...................................................................................................................................... ...................................................................................................................................... ...................................................................................................................................... ...................................................................................................................................... ...................................................................................................................................... ...................................................................................................................................... ...................................................................................................................................... ...................................................................................................................................... ...................................................................................................................................... ...................................................................................................................................... ...................................................................................................................................... LJ Technical Systems
33
An Introduction to Amplitude Modulation Chapter 2
2.8 2.8
AT02 Student Workbook
Co Con nstr stru uctin cting g th thee Am Ampl plit itu ude Mo Modu dula late ted d Wav Wavefo eform rm It is often difficult to see how the AM carrier wave can actually consist of the carrier and the two side frequencies, all of which are radio frequency signals - there is no audio signal present at all. In appearance, the AM carrier wave looks more likely to consist of the carrier frequency and the incoming information signal. Figure 26 shows this situation:
20V 15V 10V 5V 0V
Carrier Carr ier wave
-5V -10V -15V -20V Upper Upp er side side fre freq. q. 5V 0V -5V
Lo Lower wer side side freq. freq. 5V 0V -5V 0
5
10
15
20
25
30
35
40
45 time
Figure 26
Here are the three radio frequency signals that form the modulated carrier wave. We are going to add the three components and (hopefully) reconstruct the modulated waveform.
34
LJ Technical Systems
AT02 Student Workbook
An Introduction to Amplitude Modulation Chapter 2
time Figure 27 An Amplitude Amplitud e Modulated Wave
2.9
Sidebands If the information signal consisted of a range of frequencies, each separate frequency will create its own upper side frequency and lower side frequency. As an example, let us imagine that a carrier frequency of 1MHz is amplitude modulated by an information signal consisting of frequencies 500Hz, 1.5kHz and 3kHz. As each modulating frequency produces its own upper and lower side frequency there is a range of frequencies present above and below the carrier frequency. All the upper side frequencies are grouped together and referred to as the upper sideband (USB) and all the lower side frequencies form the lower sideband (LSB).
LJ Technical Systems
35
An Introduction to Amplitude Modulation Chapter 2
AT02 Student Workbook
This amplitude modulated wave would have a frequency spectrum as shown in Figure 28 below:
1MHz Carrier Lower Low er Sideb Sideban and d
This dia iag gram is not drawn to scale le..
Uppe Up perr Side Sideba band nd
Amplitude
0
0.997
0.9985 0.9 985 0.9995 0.9995 1.0005 1.0015
1.003
Frequenc Freq uency y (MH (MHz) z)
Figure 28 Frequency Spectrum Showing Upper and Lower Sidebands
Because the frequency spectrum of the AM waveform contains two sidebands, this type of amplitude modulation is often called a double-sideband transmission, or DSB.
2.10 2.10 Po Powe werr iin n tth he S Sid ideb eban ands ds The modulated carrier wave that is finally transmitted contains the original carrier and the sidebands. The carrier wave is unaltered by the modulation process and contains at least two-thirds of the total transmitted power. The remaining power is shared between the two sidebands. sidebands. The power distribution depends on the depth of modulation used and is given by: Total power =
2 N is the depth of modulation. ( carrier power )1 + where N is 2
Example: A DSB AM signal with a 1kW carrier was modulated to a depth of 60%. How much power is contained in the upper sideband? (i) (i) Start Start wi with th thee fform ormul ula: a: Total power =
36
2 N is the depth of modulation. modulation. ( carrier power )1 + where N is 2 LJ Technical Systems
AT02 Student Workbook
An Introduction to Amplitude Modulation Chapter 2
(ii) Insert all the figures figures that we know. This This is is the 1000 for the carrier power and 0.6 for the modulation depth. We could have used the figure 60% instead of 0.6 but this way makes the math slightly easier. Total power = (1000) 1 +
0.62 2
(iii) Remove the brackets.
Total power = (1000)1 +
0 .36 2
= 1000 × (1 + 0.18) = 1000 × 1.18 = 1180 W (iv) The carrier power was 1000W and the total power of the modulated wave is 1180W so the two sidebands must, between them, contain the other 180W. The power contained in the upper and lower sidebands is always equal and so 180 = 90W . each must contain 2 The greater the depth of modulation, the greater is the power contained within the sidebands. The highest usable depth of modulation is 100% (above this the distortion becomes excessive). Since at least twice as much power is wasted as is used, this form of modulation is not very efficient when considered on a power basis. The good news is that the necessary circuits at the transmitter and at the receiver are simple and inexpensive to design and construct.
Notes: ...................................................................................................................................... ...................................................................................................................................... ...................................................................................................................................... ...................................................................................................................................... ...................................................................................................................................... ...................................................................................................................................... LJ Technical Systems
37
An Introduction to Amplitude Modulation Chapter 2
AT02 Student Workbook
2.11 Practi Practical cal Exer Exercise: cise: The Dou Double ble Side Sideban band d AM Wavefo Waveform rm The frequency frequency and peak-to-peak voltage of the carrier are: ................... ......... .................... ................. ....... ................................. ............... ................................... .................................. .................................. .................................. .................................. .......................... ......... The frequency and peak-to-peak voltage of the t he information information signal signal are: ................... .......... ......... ................................. ............... ................................... .................................. .................................. .................................. .................................. .......................... ......... Record the AM waveform at tp3 in Figure 30 below.
1.2 0.8 0.4 Volts
0V -0.4 -0.8 -1.2 0
0.2
0. 0 .4 0.6 0.8 Time (milliseconds)
1.0
Figure 30 The AM Waveform at tp3 on ANACOM 1/1
The effects of adjusting the AMPLITUDE PRESET and the FREQUENCY PRESET in the AUDIO OSCILLATOR OSCILLATOR are: .................. ........ ................... .................. ................... ................... ........... .. ................................ ............... ................................. ................................. .................................. ................................. ................................. ............................. ............ ................................ ............... ................................. ................................. .................................. ................................. ................................. ............................. ............ ................................ ............... ................................. ................................. .................................. ................................. ................................. ............................. ............
38
LJ Technical Systems
AT02 Student Workbook
DSB Transmitter and Receiver Chapter 3
Chapter 3 DSB Transmitter and Receiver
3.1 3.1
Th Thee D Do ouble ble S Sid ideb eban and dT Tra ran nsm smit itte terr
Informa Inf ormatio tion n Signal Signal Audio
Antenna
Oscillator
Output Amplifier
Modulator
Carrier
AM Waveform aveform
Generator
Amplified Amplifi ed Output Output Signal
Carrier Carr ier Wave Figure 31 An Amplitude Modulated Modul ated Transmitter
The transmitter circuits produce the amplitude modulated signals that are used to carry information over the transmission path to the receiver. The main parts of the transmitter are shown in Figure 31.
LJ Technical Systems
39
DSB Transmitter and Receiver Chapter 3
AT02 Student Workbook
In Figures 31 and 32, we can see that the peak-to-peak voltages in the AM waveform increase and decrease in sympathy with the audio signal.
Informati Info rmation on sig signal nal
Amplitude Ampli tude modu modulated lated wave
Thee env Th envelo elope pe
Figure 32 The Modulation Envelope En velope
To emphasize the connection between the information and the final waveform, a line is sometimes drawn to follow the peaks of the carrier carr ier wave as shown in Figure 32. This shape, enclosed by a dashed line in our diagram, is referred to as an ‘envelope’, ‘envel ope’, or a ‘modulation ‘modulation envelope’. envelope’. It is important important to appreciate that t hat it is only a guide to emphasize the shape of the AM waveform. We will now consider the action of each circuit as we follow the route taken by the information inf ormation that we have chosen to transmit. transmit. The first first task t ask is to get hold of the information information to be transmitted. transmitted.
3. 3.2 2
The In Info form rmat atio ion n Sign Signal al In test situations it is more satisfactory to use a simple sinusoidal information signal since its attributes are known and of constant value. We can then measure various characteristics of the resultant AM waveform, such as the modulation depth for example. Such measurements would be very difficult if we were using a varying signal sign al from an external source such as a broadcast station. The next step is to generate the carrier wave.
40
LJ Technical Systems
AT02 Student Workbook
3.3
DSB Transmitter and Receiver Chapter 3
The Carrier Wave The carrier wave must meet two t wo main criteria. It should be of a convenient frequency to transmit over the communication path in use. In a radio link transmissions are difficult to achieve at frequencies less than 15kHz and few radio links employ frequencies above 10GHz. Outside of this range the cost of the equipment increases rapidly with very few advantages. Remember that although 15kHz is within the audio range, we cannot hear the radio signal because it is an electromagnetic wave and our ears can only detect waves which whi ch are due to changes of pressure. The second criterion is that the carrier wave should also be a sinusoidal waveform. Can you see why? A sinusoidal signal contains only a single frequency and when modulated by a single frequency, will give rise to just two side frequencies, the upper and the lower side frequencies. However, if the sinewave were to be a complex wave containing many different frequencies, each separate frequency component would generate its own side frequencies. The result is that the overall bandwidth occupied by the transmission would be very wide and, on the radio, would cause interference with the adjacent stations. In Figure 33 overleaf, a simple case is illustrated in which the carrier only contains three frequency components modulated by a single frequency component. Even so we can see that the overall bandwidth has been considerably increased.
LJ Technical Systems
41
DSB Transmitter and Receiver Chapter 3
AT02 Student Workbook
Carrier
Amplitude
0
A si sinu nuso soida idall Ca Carr rrier ier Wav avee
Frequency Total bandwidth
Carrier Amplitude
Frequency
0
Total bandwidth If the carri carrier er wa wave ve con conta taine ined d se seve veral ral freq frequen uenci cies es,, each each wo woul uld d pr prod odu uce it itss ow own n side side fre requ quen enci cies es.. Figure 33
On ANACOM 1/1, the carrier wave generated is a sinewave of 1MHz. Now we have the task t ask of combinin combining g the information information signal and and tthe he carrier carr ier wave to produce amplitude amplitude modulation. modulation.
42
LJ Technical Systems
AT02 Student Workbook
3.4
DSB Transmitter and Receiver Chapter 3
The Modulator There are many different designs of amplitude modulator. They all achieve the same result. The amplitude of the carrier is increased and decreased in sympathy with the incoming information signal as we saw in Chapter 2.
Informat Info rmation ion Sig Signal nal
Modulator
AM Waveform aveform
Carrier Carri er Wave Figure 34 Modulation of Information Signal and Carrier Wave
The signal is now nearly ready for transmission. If the modulation process has given rise to any unwanted frequency components then a bandpass filter can be employed to remove them.
3.5 3.5
Ou Outp tput ut Am Ampl plif ifie ierr ((or or Po Pow wer Am Ampl plifi ifier er)) This amplifier is used to increase the strength of the signal before being passed to the antenna for transmission. The output power contained in the signal and the frequency of transmission are the two main factors that determine the range of the transmission.
LJ Technical Systems
43
DSB Transmitter and Receiver Chapter 3
3.6
AT02 Student Workbook
The A An ntenna An electromagnetic wave, such as a light ray, consists of two fields, an electric field and a magnetic field. These two fields are always at right angles to each other and move in a direction that is at right angles to both the magnetic and the electric fields, this is shown in Figure 35. y
x
Electric Field
Antenna
z
This show This showss the the elec electr tric ic fiel field d mov ovin ing g ou outt fr fro om the ante ntenn nnaa. In this this exam exampl plee the the elec electr tric ic fi fiel eld d is vert vertica icall becau because se the the an anten tenna na is positioned vertically (in the dire direct ctio ion n show shown n by y) y)..
y
x
Magnet Mag netic ic Field Field
Antenna
y
z
The magn The magnet etic ic fiel field d is alwa always ys at righ rightt angl angles es to the the elec electr tric ic fiel field d so in this his cas case, it is po possitio tione ned d ho hori rizo zont ntal ally ly (i (in n the direc directi tion on sho how wn by x).
x
Antenna
Electromagnetic Wave
z
In an elect electrom romag agne neti ticc wa wave ve both fields exist together and the they mov ovee at the the spe speed of ligh lightt in a dire direcction ion that that is at righ rightt angl angles es to bo both th fiel fields ds (s (sho hown wn by the the arro arrow w labe labele led d z). z).
Figure 35 An Electromagnetic Electromagneti c Wave
The antenna converts the power output of the Output Amplifier into an electromagnetic wave. How does it do this? 44
LJ Technical Systems
AT02 Student Workbook
DSB Transmitter and Receiver Chapter 3
The output amplifier causes a voltage to be generated along the antenna thus generating a voltage difference and the resultant electric field between the top and bottom. This causes an alternating movement movement of electrons on the transmitting transmitting antenna that is really anan ACalternating current. Since an electric always has a magnetic field associated with it, magnetic field iscurrent produced. The overall effect is that the output amplifier has produced alternating electric and magnetic fields around the antenna. The electric and magnetic fields spread out as an electromagnetic wave at the speed of light (3 x 10 8 meters per second). For maximum maximum efficiency the antenna should be of a precise length. The optimum opt imum size of antenna for most purposes is one having an overall length of one quarter of the wavelength of the transmitted signal. This can be found by: λ =
v
where where v = speed speed of light, light, λ = wavel waveleng ength th and
f
f = frequen frequency cy in Hertz Hertz
In the case of the ANACOM 1/1, the transmitted carrier is 1MHz and so the ideal length of antenna is: λ =
3 × 108
1 × 10 6 300m λ = 300m
One quarter of this wavelength wavelength would be 75 meters (about 245 feet). We can now see that the antenna provided on the ANACOM 1/1 is necessarily less than the ideal size!
3.7
Polarization If the transmitting antenna is placed vertically, the electrical field is vertical and the magnetic field is horizontal (as seen in Figure 35). If the transmitting antenna is now moved by 90° to make it horizontal, the electrical field is horizontal and the magnetic field becomes vertical. By convention, we use the plane of the electric field to describe the orientation, or polarization, of the em (electromagnetic) wave. A vertical transmitting antenna results in a vertically polarized wave, and a horizontal one would result in a horizontally polarized em wave.
LJ Technical Systems
45
DSB Transmitter and Receiver Chapter 3
3.8
AT02 Student Workbook
The DSB Receiver The em wave from the transmitting antenna will travel to the receiving antenna, carrying the information with it.
Antenna
RF Amp Amplif lifier ier
Mixer
IF A m mp plifier
IF Amplifier
Diode Detector
AF Amp Amplifi lifier er Loudspeaker
Local Oscillator
Figure 36 A Superheterodyne Receiver
We will continue to follow our information signal as it passes through the receiver.
3.9
The R Reeceiv eiving A An ntenna The receiving antenna operates in the reverse mode to the transmitter antenna. The electromagnetic wave strikes the antenna and generates a small voltage in it. Ideally, the receiving antenna must be aligned to the polarization of the incoming signal so generally, a vertical transmitting antenna will be received best by using a vertical receiving antenna. The actual voltage generated in the antenna is very small - usually less than 50 millivolts and often only a few microvolts. The voltage supplied to the loudspeaker at the output of the receiver is up to ten volts. We clearly need a lot of amplification.
46
LJ Technical Systems
AT02 Student Workbook
DSB Transmitter and Receiver Chapter 3
3.10 3.1 0 Th Thee Ra Radio dio Freq Freque uenc ncy y (RF) (RF) A Amp mplif lifier ier The antenna not only provides very low amplitude input signals but it picks up all available transmissions at the same time. This would mean that the receiver output would include all the various stations on top of each other, which would make it impossible to listen to any one transmission. The receiver circuits generate noise signals that are added to the wanted signals. We hear this as a background hiss and is particularly noticeable if the receiver is tuned between stations or if a weak station is being received. The RF amplifi amplifier er is the first stage of o f amplifi amplification. cation. It has to ampli a mplify fy the incoming signal above the level of the internally generated noise and also to start the process of selecting the wanted station stat ion and rejecting rejecting the unwanted ones.
Notes: ...................................................................................................................................... ...................................................................................................................................... ...................................................................................................................................... ...................................................................................................................................... ...................................................................................................................................... ...................................................................................................................................... ...................................................................................................................................... ...................................................................................................................................... ...................................................................................................................................... ...................................................................................................................................... ...................................................................................................................................... ...................................................................................................................................... ...................................................................................................................................... ...................................................................................................................................... LJ Technical Systems
47
DSB Transmitter and Receiver Chapter 3
AT02 Student Workbook
3.11 Selecti ctivity A parallel tuned circuit has its greatest impedance at resonance and decreases at higher and lower frequencies. If the tuned circuit is included in the circuit design of an amplifier, it results in an amplifier that offers more gain at the frequency of resonance and reduced amplification above and below this frequency. This is called selectivity.
5 4 Amplifier gain
Selectivity Selectiv ity of the amplifi amplifier er
3 2 1 0
Strength Streng th of received stations
Frequency (kHz)
10mV 0 800
810
820
40
strength after after th thee amplifier in mV
840
Frequency (kHz)
We have have tu tune ned d th thee rece receiv iver er to th this is station
50 Signal
830
30 20 10 0
800
810
820
830
840
Frequency (kHz)
Figure 37
In Figure 37 we can see the effects of using an amplifier with selectivity. 48
LJ Technical Systems
AT02 Student Workbook
DSB Transmitter and Receiver Chapter 3
The radio receiver is tuned to a frequency of 820kHz and, at this frequency, the amplifier provides a gain of five. Assuming the incoming signal has an amplitude of 10mV as shown, its output at this frequency would be 5 x 10mV = 50mV. The stations being received at 810kHz andresult 830kHz each have of = one. WithThe the same amplitude of 10mV, this would in outputs of 1axgain 10mV 10mV. stations at 800kHz and 840kHz are offered a gain of only 0.1 (approx.). This means that the output signal strength would be only 0.1 x 10mV = 1mV. The overall effect of the selectivity is that whereas the incoming signals each have the same amplitude, amplitude, the outputs out puts vary between 1mV and 50mV 50mV so we can select, or ‘tune’, the amplifi amplifier er to t o pick out the desired station. stat ion. The greatest amplification occurs at the resonance frequency of the tuned circuit. This is sometimes called the center frequency. In common with nearly all radio receivers, ANACOM 1/2 adjusts the capacitor value by means of the TUNING control to select various signals.
3.12 3.12 Th Thee Lo Loca call Os Osci cill llat ator or This is an oscillator producing a sinusoidal output similar to the carrier wave oscillator in the transmitter. In this case however, the frequency of its output is adjustable. The same tuning control is used to adjust the frequency of both the local oscillator and the center frequency of the RF amplifier. The local oscillator is always maintained at a frequency that is higher, by a fixed amount, than the incoming RF signals. The local oscillator frequency therefore follows, or tracks, the RF amplifier frequency. This will prove to be very useful, as we will see in the next section.
3.13 3.1 3 Th Thee Mi Mixe xerr (o (orr Fr Frequ equen ency cy Cha Chang nger) er) The mixer performs a similar function to the modulator in the transmitter. We may remember that the transmitter modulator accepts the information signal and the carrier frequency, and produces the carrier plus the upper and lower sidebands. LJ Technical Systems
49
DSB Transmitter and Receiver Chapter 3
AT02 Student Workbook
The mixer in the receiver combines the signal from the RF amplifier and the frequency input input from the local oscillator to t o produce pr oduce three t hree frequencies: frequencies: (i) A ‘difference’ frequency of local oscillator frequency - RF signal frequency. (ii) A ‘sum’ frequency equal to local oscillator frequency + RF signal frequency. (iii) A component at the local oscillator frequency. Mixing two signals to produce such components is called a ‘heterodyne’ process. When this is carried out at frequencies above the audio spectrum, called ‘supersonic’ frequencies, the type of receiver is called a ‘superheterodyne’ receiver. This is normally abbreviated to ‘superhet’. It is not a modern idea having been invented in the year 1917.
To IF ampl amplif ifie ier r
From From RF am ampl plif ifie ier r Mixer
From local osci oscillat llator or Figure 38 The Mixer
In Section 3.12, we saw how the local oscillator tracks the RF amplifier so that the difference between the two frequencies is maintained at a constant value. In ANACOM 1/2 this difference is actually 455kHz. As an example, if the radio is tuned to receive a broadcast station transmitting at 800kHz, the local oscillator will be running at 1.255MHz. frequency is 1.255MHz - 800kHz = 455kHz.
The difference difference
If the radio is now retuned to receive a different station being broadcast on 700kHz, the tuning control re-adjusts the RF amplifier to provide maximum gain at 700kHz and the local oscillator to 1.155MHz. The difference frequency is still maintain mai ntained ed at the required r equired 455kHz.
50
LJ Technical Systems
AT02 Student Workbook
DSB Transmitter and Receiver Chapter 3
This frequency difference therefore remains constant regardless of the frequency to which the radio is actually tuned and is called the intermediate frequency (IF).
Loca osc ator frequency Amplitude
IF frequency
0
455
RF frequency
800
1255
Frequency (kHz)
Figure 39 A Superhet Receiver Receiv er Tuned to 800kHz 800k Hz
Note: In Figure 39, the local oscillator output is shown larger than the IF and RF frequency components, this is usually the case. However, there is no fixed relationship between the actual amplitudes. Similarly, the IF and RF amplitudes are shown as being equal in amplitude but again there is no significance signif icance in this.
3.14 3.14 Im Imag agee Fr Freq equ uenci encies es In the last section, we saw we could receive a station being broadcast on 700kHz by tuning tuning the local oscillator oscillator to a frequency of 1.155MHz 1.155MHz thus giving giving the difference difference (IF) frequency of the required 455kHz. What would happen if we were to receive another station broadcasting on a frequency of 1.61MHz? This would also mix with the local oscillator frequency of 1.155MHz to produce the required IF frequency of 455kHz. This would mean that this station would also be received received at the t he same same time as our wanted one at 700kHz. Station 1: Frequency 700 700 kHz, Local oscillator oscillator 1.155MHz, IF = 455kHz LJ Technical Systems
51
DSB Transmitter and Receiver Chapter 3
AT02 Student Workbook
Station 2: Frequency 1.61MHz, 1.61MHz, Local oscillator 1.155MHz, IF I F = 455kHz An ‘image frequency’ is an unwanted frequency that can also combine with the Local Oscillator Oscillator output out put to create the t he IF frequency. frequency. Notice how the difference difference in frequency between the wanted and a nd unwanted stations st ations is twice the IF frequency. In the ANACOM 1/2, it means that the image frequency is always 910kHz above the wanted station. This is a large frequency difference and even the poor selectivity of the RF amplifier is able to remove the image frequency unless it is very strong indeed. In this case it will pass through the receiver and will be heard at the same time as the wanted station. Frequency interactions between the two stations tend to cause irritating whistles from the loudspeaker.
3.15 Int Intermed ermediate iate Frequ Frequency ency Amp Amplifie lifiers rs (IF Ampl Amplifiers ifiers)) The IF amplifier in this receiver consists of two stages of amplification and provides the main signal amplifi amplification cation and selectivity. select ivity. Operating at a fixed IF frequency means that the design of the amplifiers can be simplified. simpli fied. If it were not for the fixed frequency, frequency, all the amplifiers amplifiers would need to be tunable across the whole range of incoming RF frequencies and it would be difficult to arrange for all the ampli amplifi fiers ers to t o keep in step as they are re-tuned. r e-tuned. In addition, the radio must select the wanted transmission and reject all the others. To do this the bandpass of all the stages must be carefully controlled. Each IF stage does not necessarily have the same bandpass characteristics, it is the overall response that is important. Again, this is something that is much more easily achieved without the added complication of making them tunable. At the final output from the IF amplifiers, we have a 455kHz wave which is amplitude modulated by the wanted audio information. The selectivity of the IF amplifiers has removed the unwanted components generated by the mixing process.
52
LJ Technical Systems
AT02 Student Workbook
DSB Transmitter and Receiver Chapter 3
3.16 3.16 Th Thee D Dio iode de De Dete tect cto or The function of the diode detector is to extract the audio signal from the signal at the output of the IF amplifiers. It performs this task in a very similar way to a halfwave rectifier converting an AC input to a DC output. Figure 40 shows a simple circuit diagram of the diode detector.
Input
Output
0V Figure 40 A Simple Diode Detector Dete ctor
In Figure 40, the diode conducts every time the input signal applied to its anode is more positive positive than the voltage on the top t op plate of the capacitor. capacitor . When the voltage falls below the capacitor voltage, the diode ceases to conduct and the voltage across the capacitor leaks away until the next time the input signal is able to switch it on again (see Figure 41). Wave avefo form rm at the outp output ut of the the dete detect ctor or Capacitor Capacit or discharges discharges Diode Diode co condu nducts cts and capacit capa citor or charge chargess 0V
0V
AM wave wavefo form rm at the the inpu inputt of the the dete detect cto or
Figure 41
LJ Technical Systems
53
DSB Transmitter and Receiver Chapter 3
AT02 Student Workbook
The result is an output that t hat contains three components: (i) The wanted audio information signal. signal. (ii) Some ripple at the IF frequency. (iii) A positive DC voltage level.
3.17 3.17 Th Thee Au Audi dio o Am Ampl plif ifie ierr At the input to the audio amplifier, a lowpass filter is used to remove the IF ripple and a capacitor blocks the DC voltage level. Figure 42 shows the result of the information signal passing through the Diode Detector and Audio Amplifier. The inp The input to the the diode iode dete detect ctor or fro from the the last last IF ampl amplif ifie ier r
Outp Ou tput ut of diod diodee dete detect ctor or incl includ udes es:: a DC lev eveel, the the audi audio o si sign gnal al,, 0V ripp ripple le at IF fr freq eque uenc ncy y Outpu Out putt after after fil filte terin ring g
0V
Figure 42
The remaining audio signals are then amplified to provide the final output to the loudspeaker.
3.18 The Aut Autom omatic atic Gai Gain n Con Control trol Circu Circuit it (AGC (AGC)) The AGC circuit is used to prevent very strong signals from overloading the receiver. It can also reduce the effect of o f fluctuations fluctuations in the received signal signal strength. The AGC circuit makes use of the mean DC voltage level present at the output of the diode detector. If the signal strength increases, the mean DC voltage level also increases. If the mean DC voltage level exceeds a predetermined threshold value, a voltage is applied to the RF and IF amplifiers in such a way as to decrease their gain to prevent overload. 54
LJ Technical Systems
AT02 Student Workbook
DSB Transmitter and Receiver Chapter 3
As soon as the incoming signal strength decreases, such that the mean DC voltage level is reduced below the threshold, the RF and IF amplifiers return to their normal operation.
At low low si sign gnaal st strren engt gth h the the AGC AG C ci circ rcui uitt ha hass no ef efffect ect
AGC OFF
T s part o t e transm ss on will wi ll ov over erlo load ad th thee rece receiv iver er and and caus causee dis disto tort rtion ion
Thresh Thr eshold old lev level el 0V
The AG The AGC C ha hass li limi mite ted d the the amplif amp lifica ication tion to pre preven ventt over ov erloa load d and and di disto stort rtio ion n
AGC ON
Threshol Thre shold d leve levell 0V Figure 43
The mean DC voltage from the detector is averaged out over a period of time to ensure that the AGC circuit is really responding to fluctuations in the strength of the received signals and not to individual cycles. Some designs of AGC circuit provide a progressive degree of control over the gain of the receiver at all levels of input signals without using a threshold level. This type is more effective at counteracting the effects of fading due to changes in atmospheric conditions. The alternative, is to employ an AGC circuit as used in ANACOM 1/2. In this case the AGC action does not come into effect until the mean value reaches the threshold value, this type of AGC circuit is often referred to as ‘Delayed AGC’. LJ Technical Systems
55
DSB Transmitter and Receiver Chapter 3
AT02 Student Workbook
3.19 Practi Practical cal Exer Exercise: cise: The DSB Tran Transmit smitter ter and Recei Receiver ver The depth of modulation modulation of the transmitter transmitter output o utput at tp13 t p13 is: .............................. .............. ................... ... ................................. ............... ................................... .................................. .................................. .................................. .................................. .......................... ......... Record the t he waveform at the output o utput o off the RF Amplif Amplifier ier (tp12).
Amplitude
0
0.2
0.4 0.6 Tim imee (m (ms) s)
0.8
1.0
Figure 45 The Output of the t he RF Amplifier at tp12
The incoming RF amplitude modulated wave is mixed with the output of the local oscillator to provide an amplitude modulated waveform at the required IF frequency. The RF carrier and its sidebands have effectively been reduced in frequency to the required IF frequency. Record the waveform at the output of the Mixer (tp20).
Amplitude
0
0.2
0.4
0.6
0.8
1.0
Tim imee (m (ms) s) Figure 46 The Output of the th e Mixer Circuit at tp20
56
LJ Technical Systems
AT02 Student Workbook
DSB Transmitter and Receiver Chapter 3
Record the t he waveform at the output o utput o off the first IF Amplifi Amplifier er (tp24). (t p24).
Amplitude
0
0.2
0.4
0.6
0.8
1.0
Tim imee (m (ms) s) Figure 47 The Output of the t he First IF Amplifier at tp24
Record the waveform at the output of the final IF Amplifier (tp28).
Amplitude
0
0.2
0.4
0.6
0.8
1.0
Tim imee (m (ms) s) Figure 48 The Output of the Second IF Amplifier at tp28
LJ Technical Systems
By comparing the signal amplitude of tp24 and tp28, the gain of the second IF amplifier can be calculated.
57
DSB Transmitter and Receiver Chapter 3
AT02 Student Workbook
The diode detector extracts the audio signal and removes, as nearly as possible, the IF signal. Record the waveform at the output of the Diode Detector (tp31).
Amplitude
0
0.2
Figure 49 The Output of the th e Diode Detector at tp31
0.4
0.6
0.8
1.0
Tim imee (ms) (ms)
We can see that the sinewave appears thicker than the original audio input signal. This is because what appears to be a sinewave is actually an envelope containing another frequency. The output signal from the detector is now passed through a low pass filter that removes all the unwanted components to leave just the audio signals.
3.20 3.20 Pra Pract ctic ical al E Exe xerc rcis ise: e: (AGC)
Op Opera erati tion on of tthe he A Au uto toma mati ticc Gai Gain n Co Con ntrol trol C Cir ircu cuit it
AGC Practical Exercise Notes: .......................................................................................................... .................................. ................. .................................. .................................. .................................. .................................. .................................. .................................. ................................... .................. .................................. ................. .................................. .................................. .................................. .................................. .................................. .................................. ................................... .................. .................................. ................. .................................. .................................. .................................. .................................. .................................. .................................. ................................... .................. .................................. ................. .................................. .................................. .................................. .................................. .................................. .................................. ................................... .................. .................................. ................. .................................. .................................. .................................. .................................. .................................. .................................. ................................... .................. .................................. ................. .................................. .................................. .................................. .................................. .................................. .................................. ................................... .................. 58
LJ Technical Systems
View more...
Comments
